Considerable evidence argues that prions are composed largely, of an abnormal isoform of the prion protein (PrP) designated PrPSc. Many experimental approaches have failed to find evidence for a scrapie-specific nucleic acid, yet the identification of distinct "strains" seriously challenges this conclusion. Distinct scrapie "strains" are characterized by specific incubation times when passaged through a particular host. New avenues of investigation into the structure and biology of prions may help resolve the conundrum of how the prion particle can carry the information required for "strains", yet no scrapie-specific nucleic acid has been identified. We recently found that incubation time phenotypes of Syrian hamster (SHa) scrapie "strains" change with passage in transgenic (SHaPrP) mice as well as in Armenian (Aha) and Chinese (CHa) hamsters. These findings give rise to an experimental approach that might begin to unravel the molecular bases of scrapie 'strains" or isolates. Using transgenetics, we plan to exploit the observation made by other investigators that passage of scrapie prions across the mouse/Sha species barrier frequently produces new or previously unrecognized "strains". The impact of single and multiple amino acid exchanges in hamster PrP molecules will be investigated with respect to: 1) scrapie incubation times for different "strains", 2) synthesis of prions with different properties, 3) spontaneous development of prion disease, 4) species barriers, and 5) neuropathologic changes, and 6) PrPSc distribution patterns in brain. From the results of these studies, we should be able to determine which amino acid residues in PrP modify the properties of prions and determine which amino acid substitutions produce changes in strains of prions. Since Tg(GSSMoPrP) m ice, which express MoPrPc with a Leu substitution at codon 101, spontaneously develop neurodegeneration and produce new "strains" of prions de novo, we shall assess the extent of prion diversity by analyzing the prions in the brains of these mice. The knowledge gained from the proposed studies may allow us to predict the properties of prions carrying specific PrP sequences and to produce then de novo from mutant PrP transgenes.